Bovine Respiratory Syncytial virus is a member of the family paramyxoviridae, and belongs to the genus of the Pneumoviruses. Paramyxoviridae belonging to this genus are human RSV, bovine RSV, ovine RSV, caprine RSV and pneumonia virus of mice. RSV are a member of the order Mononegavirales, i.e. the virus is a non-segmented negative strand RNA virus. The overall genomic organization of the non-segmented negative stranded RNA of the viruses belonging to the genus Pneumoviruses is comparable. The RNA consists of 10 genes, encoding eleven proteins, and has a length of about 15.2 kilobases. The RSV-proteins include two non-structural proteins (NS1 and NS2), four RNA-associated proteins, the nucleoprotein N, phosphoprotein P, the large, catalytic subunit L of the RNA-polymerase, a transcription elongation factor encoded by the first of two overlapping open reading frames of the M2 gene, and three envelope-associated proteins; the fusion protein F, the attachment protein G and the small hydrophobic protein SH. One characteristic shared by all pneumoviruses is the fact that they cause infections of the upper and (in most cases) lower respiratory tract. The human RSV has a world wide distribution and has been found to be the major pediatric viral respiratory tract pathogen. Infection of bovine species with BRSV is highly comparable to HRSV infection in many respects, i.a. in the sense that it mainly causes disease in young animals. (For a review, see Van der Poel et al.; J. Inf. 29: 215-228 (1994)). Although re-infections occur frequently in both species, in cattle they usually do not cause clinical signs (Kimman and Westenbrink, Archives of Virology 1990, 112, 1-25) which suggests that a natural infection protects against clinical signs after reinfection. Mortality varies between 1% and 30%, depending on various parameters, such as virulence of the infecting strain, climate, level of animal care and occurrence of secondary infections. (Stott et al., J. Hygiene 85: 251-261 (1980), Verhoeff, et al. Vet. Rec. 115: 488-492 (1984)). The morbidity is very high (Baker et al., Am. J. Vet. Res. 46: 891-892 (1985), Baker et al., Vet. Clin. N. Am.: Food Animal Practice 1: 259-275 (1985)). Mortality due to BRSV-infection including those cases in which BRSV infection is followed by infection with other pathogens is very high, and thus the economical losses world-wide are consequently very high.
It is clear, that efficacious and reliable vaccines for both the protection against human RSV and bovine RSV are highly wanted, but vaccine development has been hampered because it is not known how a protective immune response can be induced without causing disease. First attempts to vaccinate children with formalin-inactivated vaccines led to enhanced disease after natural infection, which suggests that vaccination may even be harmful (Anderson et al., Journal of Infectious Diseases 1995, 171, 1-7). It is known, however, that antibodies against two major surface proteins, F (a fusion protein) and G (an attachment protein), play a key role in protection (Kimman and Westenbrink, Archives of Virology 1990, 112, 1-25). However, so far vaccines based on the F- or G-protein, i.e. subunit vaccines, have not been disclosed in the literature.
One way to mimic the natural infection, c.q. to efficiently trigger the host's defense mechanism is to develop a live attenuated vaccine. Such a vaccine mimics the natural triggering of the immune system, whereas due to its attenuated characteristics, it does not induce the severe clinical signs caused by the wild-type virus. The attenuated character is usually obtained by mutating a gene that on the one hand plays a role in virulence, but on the other hand is not essential for viral infection and replication, and moreover plays no role in the induction of immunity.
Live attenuated vaccines must, as closely as possible, mimic the native RSV as far as their infection behavior is concerned. One of the main characteristics of native Respiratory Syncytial Virus is that shortly after infection it causes formation of large syncytia. Formation of these syncytia, the result of large scale cell fusion, requires the presence of the F-, the G- and the SH-protein. (Heminway, B. R. et al., Virology 200: 801-805 (1994). Expression of individual F-, SH- or G-proteins does not result in syncytia-formation. Co-expression of both the F- and SH-gene, or of both the F- and G-gene gives only very low level cell-fusion (Pastey, M. K. and Samal, S. K., J. Gen. Virol. 78: 1885-1889 (1997)). Therefore, it could be assumed that an attenuated virus suitable for use as a vaccine closely mimicking the natural infection should in principle express the proteins F, G and SH. Deletion of the gene encoding either protein G or SH results in lack of significant syncytium formation as mentioned above. Moreover, deletion of G, known to be a significant immunogen would be undesirable anyway for a vaccine virus. Therefore, if an attenuated vaccine virus is needed, a mutant RSV lacking either the SH-gene or the G-gene is not a logic choice. For these reasons, it is clear that a mutant virus that lacks even both the G-protein and the SH-protein would be the most unattractive choice for use in vaccines, because:                1) it does not form syncytia, and thus lacks one of the most characterizing features of native RSV        2) it lacks one of the two immunodominant antigens.        
Karron described a human RSV lacking both the SH-gene and the G-gene (Karron et al., Proc. Natl. Aced. Sci. 94: 13961-13966 (1997)). It was tested in patients for its suitability as a vaccine virus. The virus was however not capable of inducing an immune response in patients. As could be predicted on the basis of the arguments given above, the fact that it lacks both these genes, makes the virus over-attenuated and therefore unsuitable for vaccination purposes.